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Sleep is as important to the human body as food and water, but most of us still don't get enough sleep. We obtain treatment for illness or injury yet we generally fail to seek help when we aren't getting enough sleep. The average length of sleep has declined from around nine hours 100 years ago to seven hours today. And the depth of sleep has also declined.

Sleep is complicated because many different factors influence the effectiveness of sleep. It's not just duration that determines the effectiveness of said sleep, but factors such as quality, frame of mind and deepness all contribute to the maximum desired outcome and even our perception of how we sleep. Many factors can play a part in the quality and quantity of our sleep and an understanding of this is essential to maximise our sleep time.

On average, a healthy person will spend around one third of their life sleeping (1). Sleep is considered a natural periodic state of rest for the mind and body, in which the eyes will usually close and consciousness is completely or partially lost resulting in a decrease in bodily movements and responsiveness to external stimuli (2). Inadequate length of good quality sleep leads to a disruption to vital biological processes resulting in a decrease in cognitive function, and mental and physical health (3). This includes impaired productivity and work performance due to a decrease in attention, judgment and responsible decision-making (4).

Insomnia and CVD

Insufficient or disrupted sleep contributes to adverse health effects. Numerous studies have also shown that even a little bit of sleep deprivation decreases efficiency and increases risk of disease, including cardiovascular disease. Some of the physical effects found from long term fatigue are heart disease, diabetes, high blood pressure, gastrointestinal disorders, depression, eating disorders and weight gain. Sleep deprivation has been shown to negatively affect endocrine (hormones) and metabolic functioning, as well as nervous system balance (5). During truncated sleep, your heart might have to work harder, constricting blood vessels and increasing blood pressure even more, which could conceivably result in a heart attack or stroke (6). A recent meta-analysis of 13 prospective studies, which included 122,501 subjects, found that insomnia determined an increased relative risk of 45% of developing or dying from cardiovascular disease during the follow-up (7).

Sleep debt has been found to be associated with the hypothalamic pituitary adrenal axis as indexed by elevated evening cortisol levels and impaired glucose tolerance (8). This indicates that poor sleep may be a risk factor for the development of insulin resistance (8). Insulin resistance syndrome or metabolic syndrome is a group of risk factors for adult onset diabetes.

Obesity Link

Obesity is another disorder linked with sleep with sleep deprivation. Many studies, including one spanning 20 years, have tested the hypothesis that sleep and obesity are linked and the majority of results show positive correlations (9). Further studies, with over 500,000 total participants via meta analysis, have supported the link between obesity and insomnia in adults and children (10). The trends of increasing BMI and reduced sleep hours appear to go hand in hand, along with sleeping troubles related to sleep apnoea often seen in obese patients. Shift workers are known to be a high-risk group for obesity. In the current study population, rotating shift workers showed a higher distribution of the highest body mass index compared with daytime workers.

Effect of Stress

While many factors can contribute to insomnia and poor sleep, undoubtedly the most important one in the 21st century is stress. Increasing research shows that there is an activation of the stress-response system and the degree of sleep disturbance experienced correlates with the level of stress-response activation (11,12). It reveals that insomniacs and others with poor sleep patterns experience more psychological stress, including scoring higher in a psychological distress survey (13) and being more sensitive to stressful conditions (14).

Unfortunately, there can be a vicious cycle of stress, poor sleep and more stress. If you are not getting enough sleep, you have to rely more and more on your alarm clock, or if you oversleep you wake up already fatigued. The stress response stimulates production of many chemicals, which elevate blood glucose levels, raise heart rate, and alter the secretion of cortisol (15,16). These increases render the body unable to relax enough to sleep restfully and without enough rest, the body will not be able to readjust to a normal circadian rhythm, prolonging the cycle of feeling stressed and restless (17). Thus, a person who is always feeling tired - even after a good night's sleep - may have overworked adrenal glands.

Cortisol levels are highest in the morning and decrease gradually throughout the day (18,19). The body develops a routine, and cortisol is secreted at roughly the same amounts at a particular time of day, depending on levels of stress. If an individual does not get enough sleep, they will produce more cortisol than they would on a normal routine day. In addition, when an individual attempts to stay awake during these hours of recovery their cortisol levels will increase rapidly (20). Studies also show that stress and elevated levels of cortisol actually inhibit the ability to fall asleep, further exacerbating stress symptoms (21).

In a study using 53 women with insomnia (and 78 controls), insomniacs had higher levels of urinary morning cortisol and also scored significantly higher on a psychological distress questionnaire than did the controls (13). A similar study found the insomniac group slept for a shorter time under high-stress conditions than under low-stress conditions, while the normal sleeper group showed no variation across conditions (14).

Greater sensitivity to stress

In another study although all subjects reported equivalent numbers of daily stressful events, people with insomnia found the impacts of these events to be more severe. Insomniacs also experienced major life events more intensely, were much more alert before bedtime, viewed their lives as more stressful, and used emotionally-based coping strategies to deal with stress more often than did the normal sleeper group. The study concluded that the way insomniacs perceive and process stressful events as being beyond their control was perpetuating their insomnia (22).

A similar study looked at physiological stress (measured as hyper arousal of the nervous system), psychological stress (measured as the daily number of intrusive stressful thoughts) and sleep complaints (23). Those people with insomnia were found to have significantly more intrusive stressful thoughts and a hyper aroused nervous system. It appears that although normal sleepers and insomniacs experience similar types of stressful inputs, in insomniacs the stress-response is more sensitive to these inputs and its activation leads to lack of sleep (24).

Sleep problems can also impact severely on productivity and the workforce. In a study of 5391 female and 1454 male employees at baseline, 21% of women and 17% of men reported frequent sleep problems. Frequent sleep problems were associated with subsequent sickness absence spells irrespective of length of absence among both women and men. Associations existed for self certified sickness absence and medically confirmed intermediate and long sickness absence spells. Occasional sleep problems were also associated with sickness absence (25).

In a follow up of this study, a U-shaped association regarding sleep hours was found; those reporting short or long sleep and reporting insomnia symptoms had a higher risk for medically-certified intermediate and long sickness absence compared to those reporting seven hours of sleep without insomnia symptoms. Also, those reporting six, seven and eight hours of sleep had a higher risk for such sickness absence, if they reported insomnia. Weak associations were also found for self certified sickness absence, and for those reporting short and long sleep without insomnia. (26).

The effects of sleep deprivation can include fatigue, reduced productivity, slower reflexes and reactions, moodiness and a lack of energy, mimicking many of the characteristics of stress. The more hours we spend awake, the more sluggish our minds become, according to the findings. One night without sleep reduces our brain's ability to take in new information by nearly 40%, due to a shutdown of brain regions during sleep deprivation. Impaired sleep function decreases the neuroplasticity in the central nervous system resulting in diminished cognitive function as impulse transmissions are impaired (27). Sleep is the regenerative time for the central nervous system and nerves, without which brain atrophy and weakened conduction can result.

Brains that are sleep deprived aren't as efficient and have to work harder. Studies show the brains of the sleep deprived desperately pumping energy into the prefrontal cortex, trying to overcome the effects of sleep deprivation.

Sleep plays an important role in consolidating memories. While we sleep, our brain orders, integrates and makes sense of things that have happened to us. Not only that, but we seem to consolidate our learning while we sleep. Without sleep, the process is badly disrupted, meaning it's difficult to lay down long-term memories and it's harder to learn new skills. Sleep deprivation causes many of the powers of focus and attention to decline, which partly explains the distracted feeling you get when tired. Sleep deprived people easily get stuck in thinking loops and, as a result, fall back on the brain's automated systems, our habits.

Perhaps it is time we valued sleep more and turned off the TV and computer earlier so we can be healthier and smarter the next day.

DISCLAIMER: Dr Peter Dingle is a researcher, educator and public health advocate. He has a PhD in the field of environmental toxicology and is not a medical doctor.

Dr Peter Dingle (PhD) has spent the past 30 years as a researcher, educator, author and advocate for a common sense approach to health and wellbeing. He has a PhD in the field of environmental toxicology and is not a medical doctor. He is Australia’s leading motivational health speaker and has 14 books in publication.